Where did this heat death of the universe idea originate? First time I’ve heard of it here in this thread.

It came from Kelvin’s work on thermodynamics in the 1850s, and has been refined by many scientists since then: https://en.wikipedia.org/wiki/Heat_death_of_the_universe

It’s also the subject of a famous short story by Isaac Asimov: http://www.thelastquestion.net/